An atom is the smallest particle that can be described as a chemical.

  • At the centre of an atom is a nucleus containing protons which are positively charged sub-atomic particles. The number of protons determines the chemical element associated with the atom. A hydrogen atom has a single proton.
  • The nucleus of an atom also contains neutrons, sub-atomic particles with a mass slightly greater than that of a proton but no positive or negative charge.
  • Surrounding the nucleus are negatively charged particles called electrons which are held in place by their attraction to the positively charged protons in the nucleus.
  • In an atom, the number of electrons matches the number of protons.
  • When an electron is added or removed from an atom it forms a charged particle called an ion.
    • A cation is a positively charged ion with fewer electrons than protons.
    • An anion is a negatively charged ion with more electrons than protons.
  • The number of protons and neutrons combined determines the atomic mass of an atom.
  • Atomic mass is a measure of the total mass of protons and neutrons in an atom and is illustrated by the ordering of elements in the periodic table.
  • In atomic theory and quantum mechanics, an atomic orbital is a mathematical function describing the location and motion of electrons around the nucleus of atoms.